This invention relates to the synthesis of a substituted mercapto-benzaldehyde in an aqueous solution, with or without an organic solvent, by reacting a benzaldehyde having a leaving group with a mercaptide in the presence of a phase transfer catalyst. In particular, it relates to the synthesis of p-methyl-mercapto-benzaldehyde (PMMBAL) from p-chloro-benzaldehyde (PCBAL).
PMMBAL is an important compound which is useful as an intermediate in making drugs and agrochemicals. It can be synthesized by reacting PCBAL with an anhydrous alkali methyl mercaptide in an organic solvent with no water present. It is difficult to prepare the anhydrous methyl mercaptide, however, and the separation of the product from the salt byproduct requires filtration and extraction, followed by removal of an organic solvent and drying. The crude product also contains commercially unacceptable amounts of side products. Therefore, to obtain a product with high purity, a careful, time-consuming vacuum distillation is necessary.